Long-term synaptic facilitation in the absence of short-term facilitation in Aplysia neurons.

Serotonin (5-HT) induces both short-term and long-term facilitation of the identified synaptic connections between sensory and motor neurons of Aplysia. Three independent experimental approaches showed that long-term facilitation can normally be expressed in the absence of short-term facilitation: (i) The 5-HT antagonist cyproheptadine blocked the induction of short-term but not long-term facilitation; (ii) concentrations of 5-HT below threshold for the induction of short-term facilitation nonetheless induced long-term facilitation; and (iii) localized application of 5-HT to the sensory neuron cell body and proximal synapses induced long-term facilitation in distal synapses that were not exposed to 5-HT and had not expressed short-term facilitation. These results suggest that short-term and long-term synaptic facilitation are induced in parallel in the sensory neurons and that the short-term process, because it is induced and expressed at the synapse, can occur locally, but the long-term process, because of its dependence on a nuclear signal, is expressed throughout the neuron.